This disclosure relates to methods for estimating fracture geometry and to articles used to facilitate the same. In particular the disclosure relates to methods for estimating the length and height of the fracture.
In the completion of wells drilled into the earth, a string of casing is normally run into the well and a cement slurry is flowed into the annulus between the casing string and the wall of the well. The cement slurry is allowed to set and form a cement sheath, which bonds the string of casing to the wall of the well. Perforations are provided through the casing and cement sheath adjacent the subsurface formation. Fluids, such as oil or gas, are produced through these perforations into the well.
It is oftentimes desirable to treat subterranean formations in order to increase the production from the well. For example, in the oil industry subterranean formations are hydraulically fractured in order to facilitate the flow of oil and/or gas into the well or the injection of fluids such as gas or water from the well into the formation. Such hydraulic fracturing is accomplished by disposing a suitable fracturing fluid within the well opposite the formation to be treated. Thereafter, sufficient pressure is applied to the fracturing fluid in order to cause the formation to break down with the attendant formation of one or more fractures therein. Simultaneously with or subsequent to the formation of the fracture, a suitable carrier fluid having suspended therein a propping agent or proppant such as sand or other particulate material is introduced into the fracture. The proppant is deposited in the fracture and functions to hold the fracture open after the fluid pressure is released. The fluid containing the proppant is of a relatively high viscosity in order to generate a wider fracture width and reduce the tendency of the propping agent to settle out of the fluid as it is injected down the well and out into the formation. High viscosity fluids increase the fracture width and allows more proppant to be transported into the fracture. It also helps control leak-off of the fracturing fluid into the walls of the fracture being created.
Certain aspects of the extent of such formation fracturing and the location of proppant materials has been ascertained by the use of radioactive tracers. The radioactive tracers have been plated or coated on propping agents or added as a liquid and injected along with the fracturing fluid. These coatings generally contain radioactive isotopes. Although the use of such radioactive tracers or coatings yields useful information, its usefulness is limited to fracture locations near the wellbore and yields little if any useful information relating to the dimensions of the fracture as it extends into the formation. The use of radioactive tracers also presents monitoring, logistical and environmental problems. The short half-lives of such tracers prevent the monitoring of the movement of such tracers in the formation fractures other than over a short time interval. Transportation and use of radioisotopes is expensive and government regulations or restrictions must be accommodated. Disposal of excess radioactive proppants can be a problem, especially in offshore operations.
It is therefore desirable to develop a method for hydraulic fracturing in which the extent of such fracturing is measured without the use of radioactive isotope tracers. It is also desirable to determine the geometry of the formation fracture and in particular the penetration or length of the fracture that extends out from the wellbore.